Open Wi-Fi Sync on your iOS device and select the computer currently running the Wi-Fi Sync desktop application.

If you're using the original Wi-Fi Sync version, you'll simply need to authorize Wi-Fi Sync on your desktop computer. This will let iTunes see the device when it's connected over Wi-Fi, letting you sync whenever the iOS Wi-Fi Sync app is open. If you're using the 2.0 beta version, you'll need to enter a passcode that the desktop version provides. Once you do that, you can sync from pretty much anywhere and can initiate sync directly from the iOS app.